1. Field of the Invention
This invention relates to the control of a drive source, and particularly to the control of a vibration type actuator that generates a driving force using vibration energy.
2. Related Background Art
There have heretofore been made many propositions regarding a control method for a vibration type actuator. In the-case of a vibration type actuator, the rotational velocity thereof is often controlled by the frequency (driving frequency) or the amplitude (driving voltage) of a frequency signal applied to a vibration member.
The relation between the driving frequency and the rotational velocity is as shown in FIG. 10 of the accompanying drawings. The rotational velocity has a characteristic in that, with respect to the resonance frequency of the vibration member as a peak, it has a gentle slope on the high frequency side and a steep slope on the low frequency side. Also, the velocity varies in accordance with the driving voltage as shown in FIG. 10.
When velocity control of the vibration type actuator is to be effected by changing the driving voltage, the frequency must be made approximate to and higher than the resonance frequency of the vibration type actuator; however, the resonance frequency also varies with changes in the environment, such as temperature and load, and therefore cannot be completely fixedly controlled
On the other hand, when velocity control is to be effected by changing the driving frequency, even if environmental factors such as temperature, load, and the like change, control conforming thereto can be effected.
As means for generating a frequency signal of any frequency, use is often made of a VCO (voltage control oscillator). The VCO can output a signal of a frequency conforming to the magnitude of a voltage input thereto, and can obtain a high power resolution of a relatively fine frequency. Therefore, if the VCO is used when the velocity of the vibration type actuator is to be controlled by controlling the frequency, stable control can be effected.
However, when an attempt is made to reduce the cost of a circuit, it is conceivable to provide a construction which does not use an analog circuit such as the VCO, but frequency-divides a high clock pulse signal using a digital circuit (a gate array or the like) to thereby generate a pulse of a frequency corresponding to the frequency of a frequency signal, and to control the gate of a switching element on the primary side of a step-up transformer to thereby drive the vibration type actuator.
The higher the frequency of the frequency-divided clock, the finer can be made the resolution power of the driving frequency. However the frequency of the clock pulses which can be used in the gate array is limited and therefore, the resolution power cannot be increased as much as by using a VCO. If an attempt is made to control the speed of the vibration type actuator only by varying the frequency, there arises the problem that velocity irregularity becomes great.
So, as means for controlling the driving speed of the vibration type actuator, there have been made several propositions for controlling it by combining control of the amplitude and frequency of an AC signal of two phases input to the actuator and the phase difference between the two phases.
For example, in Japanese Laid-Open Patent Application No. 64-85587, there is proposed a technique of sweeping the driving frequency of a vibration type actuator from a higher frequency toward a lower frequency, fixing the frequency when the velocity has reached a predetermined velocity and then controlling a driving voltage or a pulse width to thereby control the velocity.
Also, Japanese Laid-Open Patent Application No. 5-328757 proposes a technique of controlling the velocity so as to become constant by controlling the pulse signal width of a pulse input to the power amplifying portion of a vibration type actuator in accordance with a driving voltage, and controlling a driving frequency so that the pulse width as the controlled result may assume a predetermined value.
Further, Japanese Laid-Open Patent Application No. 6-237584 proposes a technique of controlling the frequency at a first step to thereby control the velocity so as to approach a target velocity when the frequency and voltage amplitude or the frequency and the phase difference of an input AC signal of two phases of a vibration type actuator are to be controlled, fixing the frequency when the velocity has reached a certain velocity range, and thereafter controlling the velocity so as to approach the target value by controlling the voltage amplitude or the phase difference of the AC signal of two phases or, on the other hand, controlling the velocity again by controlling the frequency when the difference from the target velocity is greater than or equal to a certain value thereafter.
However, according to the technique proposed by Japanese Laid-Open Patent Application No. 64-85587, the driving frequency is fixed during a start-up period, and this leads to the undesirable possibility that the fixed frequency diverges from the resonance frequency, whereby a predetermined velocity (number of revolutions) cannot be output. Also, when conversely the fixed frequency becomes too close to the resonance frequency, and particularly when the target velocity is low, there is the undesirable possibility that the driving voltage may be reduced to a level at which the vibration type actuator stops.
Also, according to the techniques proposed by Japanese Laid-Open Patent Application No. 5-328757 and Japanese Laid-Open Patent Application No. 6-237584, after the velocity has reached a steady velocity, control is effected chiefly by controlling the driving voltage and, when due to changes in temperature, load, etc., control becomes impossible by varying only the driving voltage, the driving frequency is changed to thereby make control by varying the driving voltage again possible. However when the generation of the frequency is performed using a digital circuit, the resolution power of the frequency becomes bad, and this leads to the undesirable possibility that the fluctuation of the velocity becomes great even if the frequency is changed by a minimum step. Particularly, when high accuracy of the number of revolutions (rotational velocity) is required as is the case in the rotation of a developing drum or the like of a copying apparatus, if the driving frequency is changed during image formation and transfer, there is the possibility that the number of revolutions of the drum will fluctuate greatly and the quality of printing is degraded.